The performance of digital handsets generally has been improving over the years with respect to functionality, performance, and battery life. Handset performance is being addressed by the introduction of 3rd generation (3G) technology, with higher data rates and better provision for new services. One key consideration is the need for the 3G networks to more than just coexist but handover active calls/data sessions with the current 2/2.5G networks. Where 3G coverage is absent, the user will need to utilize the 2/2.5G network, bringing with it the requirement for the phone to support both radio access technologies (RATs). For this reason, 3G phones often support at least two RATs.
Not only must the phone be able to search for either type of network at power-up, but also to re-select the network type when moving out of the 3G coverage area. During a call, that call must be “handed over” from one network type to the other without the user being aware of it happening. Testing this capability is potentially a complex process, involving many different network elements. Intersystem handover is the process of maintaining a phone connection while moving from one cell to another of a different radio access technology. A number of solutions have been developed to overcome the problems specifically associated with intersystem handover between different RATs. These include the use of dual-mode user equipment (UE), compressed-mode channel measurements, cell re-selection, cell change order, and inter-RAT handover. In addition, by embedding messages in one RAT as if it was of the other has made it possible to leave networks which are specific to a particular RAT unchanged.
In an intersystem handover process, the UE must perform signal strength/quality measurements of the intended RAT. Because the UE is occupied with the existing RAT and the measurement process must be performed simultaneously, a transmission gap has been made available to do inter-frequency measurements. In Global System for Mobile communication (GSM), such idle gaps exist, as it's a time division multiple access (TDMA)-based technology. The Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) system is a code division multiple access (CDMA)-based system and measurement gaps can be created by using a compressed mode. Measurement reporting is a major element of the intersystem handover process. To initiate the procedure, the UTRAN may transmit a measurement control message or information to the UE, including the measurement identification (ID) and type of measurement to initiate. When the reporting is completed, the UE sends a measurement reporting message back to the UTRAN with the measurement ID and the results. The message is broadcast in idle mode within the system information. When the UE monitors base stations on other RATs, the UTRAN must direct the specific measurement needed to fulfill the requested handover.
In long term evolution (LTE) systems, a UE in active mode (e.g. LTE_ACTIVE) measures neighbour cells that are included in inter-frequency and inter-RAT system cells as well as intra-frequency cells to perform handover. Hence, a measurement gap is necessary to turn to neighbour cells for LTE inter-frequency or inter-RAT measurements (i.e. monitoring GSM/UMTS). To ease signalling and the specification handling, one single measurement gap duration (e.g. 6 ms) could be used for example for LTE inter-frequency, UMTS and GSM measurements. Such a single gap has advantages for signalling and implementation making it less complex.
FIG. 4 shows a schematic representation of a data stream with intermediate transmission gaps G1, G2 arranged between transmission periods T1 to T3. In case that several RATs are configured in the measurement control information, a uniform handling in the UEs needs to be ensured to have similar behaviour of terminals. Furthermore, mobility testing is believed to be more difficult, i.e. what is the amount of gaps to be considered to be used for a specific technology in case that more than one technology is present, if not indicated?
Autonomous usage of measurement gaps by the UE would not allow any control from network side and in case of different implementations of the UE measurement state machine in different terminals, different performance may be seen from terminal to terminal which is a drawback from network perspective only being able to rely on the minimum performance.
In UMTS measurement gaps can be provided on a flexible basis concerning size and periodicity, which leads to an increased complexity. In addition each measurement opportunity (compressed mode gap) can have its designated purpose for e.g. FDD (frequency division duplex) measurement, GSM carrier RSSI (received signal strength indicator), BSIC (base station identity code) identification, GSM BSIC re-confirmation etc., so that scheduling of measurement gaps becomes very complex and inflexible. The exact usage of gaps among technologies is clearly indicated, which allows exact prediction of the maximum time it will take for a UE to identify a certain cell of a RAT and corresponding definition of performance requirements.
However, considering the required amount of signaling such an approach can be disadvantageous, e.g., for LTE system.